This invention relates to a modulating controller for controlling two operation terminals, and more particularly to a device to suppress disturbance to a process when a control signal is switched or when a bias is set.
FIG. 1 shows a conventional modulating controller, and this modulating controller controls operation terminals each disposed in a branched channel. The operation terminal is a flow control valve for example.
Liquid flows in this channel in the directions from A to B. This channel is parted into branch lines 2A, 2B on its way. The branch lines 2A, 2B are provided with operation terminals 3A, 3B respectively. In the joined channel, orifice 4 is provided. Differential pressure before and after the orifice is detected by sensor 5. This detected signal is subjected to the calculation of extraction of the square root, and inputted into modulating controller 1 as a measured value of this flow rate. The modulating controller 1 conducts a control-operation so that the operation terminals 3A, 3B share a half of the flow rate needed, and the operation terminal 3A control signal and the operation terminal 3B control signal are outputted to the corresponding operation terminals 3A, 3B to control the flow rate of the channel at a certain value.
FIG. 2 shows a construction of a specific modulating controller 1. The modulating controller 1 comprises setting signal generator means 6, difference operation means 7, PID operation means 8, and two H/A station 9A, 9B.
The difference operation means 7 operates a difference signal between the detection signal of the sensor 5 and the set signal of the setting signal generator means 6. This difference signal is entered into the PID operation means 8. The PID operation means 8 conducts operation treatments of proportion, integral and differential based on this difference signal, and outputs PID control signal. This PID control signal becomes the operation terminal 3A control signal and the operation terminal 3B control signal via two H/A station 9A, 9B. According to the operation terminal 3A control signal and the operation terminal 3B control signal, the corresponding operation terminals 3A, 3B are controlled.
When the operation terminals 3A, 3B are manually operated, a manual section provided on the H/A station 9A, 9B is operated. When an automatic operation signal is inputted into the H/A station 9A, 9B, the operation terminals 3A, 3B are controlled to be opened or closed by the PID control signal. Thus, the H/A station 9A, 9B have functions to switch to a signal by manual operation or PID control signal. Therefore, the operation terminals 3A, 3B can operate separately by manual operation other than PID control signal.
When a half of the flow rate needed is shared to control by the operation terminals 3A, 3B, as a rule, the operation terminals 3A, 3B use the same equipment. Even if the same equipment is used, characteristics may vary slightly depending on each equipment. In such a case, in order to compensate for the characteristics of the equipment, a bias setting is made to one of the operation terminal control signals.
FIG. 3 is an example of a control block diagram having bias setting means 10.
The system on the operation terminal 3B side has a bias function. The bias setting means 10 has a plus or minus bias setting signal set. Addition means 11 adds the bias setting signal to the PID control signal and outputs.
For example, in the plant structure shown in FIG. 1, at either of the operation terminals 3A, 3B, the needed flow rate is flown, and the other operation terminal is determined to be a backup. In such a structure, the valve of each operation terminal to be used is a valve which can flow the base flow or more. In this structure, one operation terminal only is automatically operated and the other operation terminal is manually operated.
When the operation terminal to make automatic operation is switched to the other operation terminal, one operation terminal is once changed to manual operation, then the other operation terminal is switched to automatic operation. Therefore, there is a period that both of them are in manual operation. In order to make it bumpless when the PID control is switched from manual operation to automatic operation, it is necessary to always tracking the current output of the operation terminal which becomes automatic operation next time to the integral element of the PID operation means of the modulating controller 1. However, since it is impossible to assume which will be automatically operated next time, it cannot be determined which output of the operation terminals 3A, 3B is tracked.
Heretofore, as a countermeasure in such a case, the devices shown in FIG. 4, FIG. 5 and FIG. 6 were used. The device of FIG. 4 has mean value operation means 13A provided. The mean value operation means 13A operates the average of the operation terminal 3A control signal and the operation terminal 3B control signal, and outputs a tracking signal to the PID operation means 8. The device of FIG. 5 is provided with high selector means 13B. The high selector means 13B selects the high values of output of the operation terminal 3A control signal and the operation terminal 3B control signal, and outputs a tracking signal to the PID operation means 8. The device of FIG. 6 is provided with low selector means 13C. The low selector means 13C selects the low values of output of the operation terminal 3A control signal and the operation terminal 3B control signal, and outputs a tracking signal to the PID operation means 8. The devices shown in FIG. 4 and FIG. 5 have a structure to make the tracking condition generator signal an ON signal when the automatic selection signal A and the automatic selection signal B are execution halt signal (OFF signal). When this tracking condition generator signal is ON signal, the tracking signal is inputted in the PID operation means 8 and tracked to the integral element of the PID operation means 8.
However, conventional devices shown in FIG. 2 through FIG. 8 give disturbance to the process system of the plant every time the operation terminal is separated from PID control or entered under PID control, and they have a problem that the plant is put under a dangerous condition according to the magnitude of disturbance.
First, conventional modulating controller 1 shown in FIG. 2 has the operation terminal 3A control signal and the operation terminal 3B control signal equal under an ordinary condition that the H/A station 9A, 9B are under automatic operation. And, two operation terminals 3A, 3B share the flow rate into a half to control so that the flow rate becomes equal to the set signal. When either of the H/A station 9A, 9B is switched from this condition to manual operation, the control input of the manual operation to the flow rate which has been shared in half by the operation terminal 3A and the operation terminal 3B gives disturbance to the process system resulting in varying the process system.
Conversely, when either of the operation terminals is returned from the manual operation to the automatic operation, disturbance is also given to the process system. Therefore, there was required the operation by a skilled operator to switch to the automatic operation after operating so that the operation terminal under automatic operation and the operation terminal under manual operation gradually become an equal output signal gradually.
Second, conventional modulating controller 1 shown in FIG. 3, when bias setting is operated by bias setting means 10, from a state that the operation terminals 3A, 3B flow the flow rate in half to control, the bias control input content disturbs the process system. Such a conventional device had a problem to give disturbance to the process system of the plant every time the bias operation was conducted.
Third, since the conventional devices shown in FIG. 4 through FIG. 8 do not always track the opening of the operation terminal which is automatically operated next time, it cannot be said to be a perfect tracking which can correspond to any case.
That is to say, in case of the device shown in FIG. 4, the mean value of the operation terminal 3A control signal and operation terminal 3B control signal is tracked. Therefore, whichever of the operation terminal 3A control signal and operation terminal 3B control signal is switched to automatic operation, bumping is made to the mean value, and automatic operation is started from the mean value.
In case of FIG. 5, a larger one of the operation terminal 3A control signal and operation terminal 3B control signal is tracked. When the larger one of the operation terminal 3A control signal and operation terminal 3B control signal is switched to automatic operation, shift is made to automatic operation without bump. Therefore, there in no problem. When a smaller one of the operation terminal 3A control signal and operation terminal 3B control signal is switched to automatic operation, it is bumped to the larger one, then automatic operation is started.
In case of the device shown in FIG. 6, a smaller one of the operation terminal 3A control signal and operation terminal 3B control terminal is tracked. When the smaller one of the operation terminal 3A control signal and operation terminal 3B control signal is switched to automatic operation, shift is made to automatic operation without bump. Therefore, there is no problem. When the larger one of the operation terminal 3A control signal and operation terminal 3B control signal is switched to automatic operation, it is bumped to the smaller one, then automatic operation is started.
This bumping between the operation terminal 3A control signal and the operation terminal 3B control signal becomes disturbance against the plant, making the plant on a dangerous condition.